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Fundamentals of Neuroscience/Electrical Currents
= Basic Properties of Electrical Currents = AIMS # To Introduce the electron # To Introduce the idea of Electron Shells # To Introduce the idea of charge of an Atom # To Introduce the idea of Ions as charged Atoms # To Introduce the idea of Amperage as Current # To introduce the idea of Ions moving in a particular direction as an Ion Current # To introduce the idea of Voltage as EMF # To introduce the idea of Capacitance Lesson Although Electricity in it's many forms is an interesting subject, in it's own right, the idea of this course is to give you enough of an understanding of how Electricity works in the Neuron to be able to make sense out of more advanced training opportunities. As such we aren't going to get into the advanced uses of electricity, or electronics, instead we will cover the basics, and then veer off from the normal approach to electricity to introduce concepts needed to understand how electricity works in neurons. The Electron At the heart of Electricity and Electronics, and even Neural Potentials, lies at least theoretically a sub-atomic particle called an electron. The electron is negatively charged, and orbits the Nucleus of the Atom, at about the speed of light. It is held in place by the electro-weak force, which is in turn caused by charge attraction between the light electron and the heavier protons that with neutrons makes up the Nucleus of the atom. It is thought that electricity happens when electrons break free of the atom they are normally part of, and wander among other atoms in the same cluster. Atomic Charge Since electrons can wander away from their original atoms, and get picked up by other atoms, the atoms that have more electrons than normal are considered to be negatively charged, and the atoms that have fewer electrons than normal are considered to be positively charged. Since electricity flows in the direction of lowest potential, Atoms that are negatively charged, will discharge their electrons to atoms that are positively charged, if the intervening atoms allow the transfer of electrons. We say that the electrons flow from the Negative charge to the positive charge, possibly since the original theory was that electricity was some sort of fluid. Ionic Charge We call an atom that is carrying a charge, whether it is negative or positive an Ion. In chemical systems based on water, many ions are created by the nature of water which levers apart many common chemicals into their component ions. If the external shell of the atom, called the Valence Shell, has an odd number of electrons, the ion usually has a negative charge, and if it has a positive number of electrons the ion usually has a positive charge. It is possible in some cases to strip off two valence electrons, in which case you end up with a double positively charged ion. Electric Transfer from Ion to Ion The Ability to allow a charge of electricity to travel from one location to another depends on the resistance of the intervening electrons to having their electrons stolen, and replaced. The looser the electro-weak forces holding onto the electrons, the less resistance to flow is found, and thus the greater the potential flow given a particular pool of electrons on one side of the system, and the pool of positively charged ions on the other. Electrical Charge is measured in Coulombs, where Principles of Electric Circuits 2nd Edition, Thomas L. Floyd, (1985,1981) Charles E. Merrill Pub. (Bell and Howell) Columbus Ohio, ISBN 0-675-20402-X *1 Coulomb = 6.25 * 1018 electrons. *The charge of a single electron is 1.6 * 10-19 Coulombs. *1 AMP is the amount of current flowing when 1 Coulomb per second passes a particular point in one second. Ion Currents Since Current is measured in Coulombs per second passing a point, it doesn't really matter whether the electrons are moving or the ions are moving. In fact it is the nature of electronics that we can postulate a flow counter to the flow of electrons that indicates the electrical force pushing the ions. We use this in welding by forcing metal ions to jump from a rod that consists of an electrical conductor surrounded by flux, to another piece of metal thus joining or building up the metal where it lands. In Neurons we often have to measure ionic currents where a specific ion is being transferred across the membrane, and thus affecting the charge stored in the neuron. Electrical Transfer from High to Low Potential In order for an electron to move from one atom to another, it requires an electro-motive force, to push it against the resistance of the intervening atoms. This force, measured in Volts, is the amount of energy symbolized by E needed to move an amount of charge symbolized by Q. V = E/Q One Volt is defined as the amount of potential difference between two points when one joule of energy is used to move one Coulomb of charge from the one point to the other. A joule is the amount of energy needed to move an object one meter against an opposing force of one newton (0.225 lb.) Given either Amperage and Voltage, voltage and resistance, or Amperage and resistance, you can calculate the third of these three factors, using the formula I=V/R which is called Ohms law. Charge Separation across a Dielectric When resistance is high enough, electrons cannot flow across the resistant material, however they are still repulsed by each other by the electro-weak force, so when a resistance that is large enough to stop electron flow is found, the electrons tend to gather on one side of the material and the positive ions tend to gather on the other, until the voltage generated across the material exceeds it's dielectric constant, and a current begins to flow. We call this tendency for high resistance to result in a charge separation, capacitance. We call the highly resistive material a Dielectric, and the charge capacity of the capacitor is directly related to the plate size on each side of the dielectric. Capacitors are measured in Farads (F) and C = Q/V 1F = 1 Coulomb / 1 Volt The Cellular Membrane as a Capacitor By transferring ions across the Cellular membrane which is made of highly resistive materials, the net effect is that of turning the membrane into the dielectric of a capacitor. As ionic currents add to or subtract from the charge building up inside the neuron, the ions line up along the cell membrane attracted to the opposite charged ions on the other side of the membrane. If an ion is moving away from its own type of charge, the capacitance of the membrane acts to speed it on its way, if it can find a pore, or ion channel to flow through. However to move towards it's own type of charge, energy must be spent in the form of voltage to pass the current across the membrane. Thus ion channels that pump ions against the charge gradient of the membrane capacitance must burn energy usually in the form of ATP. As the charge builds up the Potential between the inside of the membrane and the outside of the membrane increases until it reaches the dielectric constant of the membrane, at which point the membrane depolarizes. Usually resulting in the firing of the cell. Thus understanding capacitance is important to understanding the nature of the cell membrane and how it impacts the firing of Neurons. References